In air-cooled heat exchangers and air preheaters used in petroleum refineries, petrochemical plants and the like as well as flue-gas desulfurization equipment used in thermal power stations and so on, not only sulfuric acid, but also a reducing acid which has a highly corrosive action, such as hydrochloric acid and so on, is generated when combustion gases are cooled. For this reason, it has been impossible to avoid the occurrence of corrosion in Fe based corrosion-resistant alloys, such as conventional low alloy steels and stainless steels.
Therefore, in recent years, in some of desulfurization equipment and the like, there have been used Ni based alloys having a markedly better corrosion resistance to sulfuric acid in comparison with Fe based alloys. Concretely, commercial Ni based alloys containing Cr, Mo and W with 20% Cr-15% Mo-4% W as a basic chemical composition, such as Hastelloy C22 and Hastelloy C276 (“Hastelloy” is a trademark), the Ni based alloy containing 16 to 27% of Cr, 16 to 25% of Mo and 1.1 to 3.5% of Ta which is disclosed in the Patent Document 1 and so on have been used.
As highly corrosion-resistant alloys, for example, in the Patent Documents 2 and 3, austenitic alloys used in waste incinerators and the like are disclosed. An austenitic stainless steel for flue-gas desulfurization equipment and for seawater service, which is excellent in crevice corrosion resistance and hot workability, is disclosed in the Patent Document 4. Moreover, austenitic stainless steels excellent in high temperature corrosion resistance, which are suitable for seawater service and for heat exchangers of incinerators, are disclosed in the Patent Documents 5 and 6.
Furthermore, an austenitic steel welded joint and a welding material, which are excellent in resistance to weld cracking and corrosion resistance to sulfuric acid, are disclosed in the Patent Document 7. Moreover, in the Patent Document 8, a Ni—Cr—Mo—Cu alloy excellent in corrosion resistance to sulfuric acid and wet-treated phosphoric acid is disclosed.    Patent Document 1: JP 8-3666 A    Patent Document 2: JP 5-195126 A    Patent Document 3: JP 6-128699 A    Patent Document 4: JP 10-60603 A    Patent Document 5: JP 2002-96111 A    Patent Document 6: JP 2002-96171 A    Patent Document 7: JP 2001-107196 A    Patent Document 8: JP 2004-19005 A